Thermally conductive silk-screenable interface material

ABSTRACT

The present invention provides a novel visible light curable composition for forming a thermally conductive interface and a method of using the same. The composition is used to promote the transfer of heat from a source of heat such as an electronic device to a heat dissipation device such as a heat sink. The composition includes an elastomeric base matrix containing a light curable catalyst, loaded with a thermally conductive filler material such as boron nitride grains or ceramic filler. After the compound is prepared, it is screen or stencil printed onto the desired surface and cured by exposure to visible light. The present invention provides a thermal interface that is bonded to the surface of the desired surface and has sufficient compressibility to allow it to overcome the voids in the mating surface to which the assembly is mounted.

BACKGROUND OF THE INVENTION

The present invention relates to an electronic material composition foruse in connection with heat generating electronic devices and a methodfor using the same. More particularly, this invention relates to a newelastomeric polymer coating material, containing thermally conductivefiller material for forming an improved thermally conductive interfacelayer, and its method of application to a heat dissipation device forthe purpose of transferring waste heat from an electronicheat-generating source.

In the electronics and computer industries, it has been well known toemploy various types of electronic device packages and integratedcircuit chips, such as the PENTIUM central processing unit chip (CPU)manufactured by Intel Corporation and RAM (random access memory) chips.These integrated circuit chips have a pin grid array (PGA) package andare typically installed into a socket, which is soldered to a computercircuit board. These integrated circuit devices, particularly the CPUmicroprocessor chips, generate a great deal of heat during operationwhich must be removed to prevent adverse effects on operation of thesystem into which the device is installed. For example, a PENTIUMmicroprocessor, containing millions of transistors, is highlysusceptible to overheating which could destroy the microprocessor deviceitself or other components proximal to the microprocessor.

In the prior art, heat sinks are commonly placed in thermalcommunication with the heat-generating device to help dissipate heattherefrom, to avoid the above-described adverse effects due tooverheating. Heat sinks are often placed and maintained in directcommunication with the heat-generating surface of the device to becooled. For example, heat sinks are commonly affixed to the top surfaceof a device and maintained in place by some type of clip or clamp unit.In operation, the heat emanates from the device and to the adjacent heatsink for dissipation into the ambient air.

However, small air gaps or voids are present between the surface of thedevice to be cooled and the heat sink member. The contact surfaces arenot perfectly flat. Due to manufacturing methods, such as machining, ofthe heat sink, which may be constructed of metal or thermally conductiveplastic, there are voids or grooving present, although very small. Also,the surface of the device to be cooled typically has similarimperfections and accompanying air gaps. As a result, when a heat sinkis mated with a surface to be cooled, precise flush communication is notpossible due to the small air gaps present therebetween and theeffectiveness of the heat sink will be degraded because the thermalconductivity of the air in the gaps is less than that of the material ofthe heat sink itself.

To address the above concerns, various interface materials have beenemployed in the prior art. In particular, organic base materials such aspolysiloxane oils or polysiloxane elastomeric rubbers and thermoplasticmaterials such as PVC, polypropylene, etc. loaded with thermallyconducting ceramics or other fillers such as aluminum nitride, boronnitride or zinc oxide have been used to impart thermally conductingproperties to the organic base material. In the case of polysiloxaneoils loaded with thermally conducting materials, these materials areapplied by smearing the heat sink or other electronic component with thethermally conducting paste and then securing the heat sink in place bymechanical means using clips or screws. In the case of polysiloxanerubbers and thermoplastic polymers, these materials are typically castin sheet form and die cut into shapes corresponding to the shape of theheat sink and heat generating device. The resulting preformed sheet isthen applied to the surface of the heat-generating surface securing theheat sink by means of clips or screws.

Thermal greases are also used to conduct heat in electronic devices. Theprior art thermal greases show superior film forming and gap fillingcharacteristics between uneven surfaces thus providing an intimatecontact between the surface of the heat sink and the surface of theheat-generating source. However, it has been found that the use ofthermal greases exhibit poor adhesions to the surfaces of the heat sinkand heat generating surface, thus effectively seeping out from betweenthe heat sink and the heat generating surface, causing air voids to formbetween the two surfaces leading to hot spots. Moreover, excessivepressure placed upon the heat sink by the mechanical fastenersaccelerates this seepage from between the heat sink and the surface ofthe heat-generating surface. It has been reported that excessive squeezeout of polysiloxane oils can evaporate and recondense on sensitive partsof the surrounding microcircuits. The recondensed oils lead to theformation of silicates thereby interfering with the function of themicroprocessor and eventually causing failure.

The precut films solve the problems associated with greases but do notprovide adequate intimate contact required for optimum heat transferencebetween the heat generating source and the heat sink. Typical precutfilms do not show the film forming capacity, as do the thermal greases.The added step of cutting preforms and manually applying the pad addscost to the assembly process. Furthermore, these types of materials showvariable performance due to variation in the thickness of the pad andthe amount of pressure applied to the thermally conducting precut film,based upon the mechanical device or action used to secure the heat sink.Further, while these known interface materials, are suitable for fillingundesirable air gaps, they are generally are less thermally conductivethan the heat sink member thus detracting from the overall thermalconductivity of the assembly.

The prior art in this area includes a silk screen printable acrylatematerial that is cured with ultra-violet wavelength light. Thesematerials have several drawbacks due to the nature of the materialsemployed. Once cured, acrylate materials tend to be rather brittle andinflexible providing poor compressibility in applications where theinterface must be held in tight contact with the heat generating devicethereby bridging small gaps. An additional drawback is the result of theUV curing requirement. UV curing requires specialty equipment includinghigh intensity actinic lamps to produce the proper radiation required tocomplete the curing process. Not only is the required equipmentexpensive To cure a product using a able

In view of the foregoing, there is a demand for an interface for a heatsink assembly that is capable of both filling the undesirable voids andimperfections in the mating surfaces of a heat sink and the object to becooled. There is a demand for an interface that can be easily applied toa surface and cured without complex equipment. There is a further demandfor a flexible interface material that can accommodate the inherentcreep associated with heat sink devices. There is also a demand for aninterface to also improve the thermal conductivity of the interfacebetween a heat sink and object to be cooled. In addition, there is ademand for an interface construction that can be installed onto existingheat sink designs in an efficient manner.

SUMMARY OF THE INVENTION

The present invention is generally directed to a novel and uniquethermally conductive interface. The present invention provides a new andimproved composition for forming a thermally conducting polymericinterface layer or film for use in electronic applications, and a methodof applying such material. A compound having an elastomeric base matrixloaded with a thermally conducting filler is used to impart thermallyconductive properties to the screen printable coating material system orgel. When cured, this thermally conducting thermal interface, beinghighly compressive, forms an intimate contact between the heat sourceand the heat sink. The material composition remains in molten formduring application to the substrate material, but includes one or morevisible light curable elastomeric materials that are cured via the useof catalysts and visible light into a highly compressible thermalinterface. Since this material is thermoset, unlike the prior art, nosqueeze out of the material can occur subsequent to curing.

After the material composition is prepared, it is either screen-printedto a film thickness onto the surface of a heat sink or other electroniccomponent, or if a thicker interface is desired, stencil printed andthen cured. The resultant film formed upon the heat sink or other devicecan be controlled to close tolerances, thereby imparting a consistentthickness and accurate placement on the desired part, thereby allowingthe uniform transfer of heat. In addition, since the film of the presentinvention is formed from an elastomeric base material, it is highlycompressible, allowing the thermally conductive interface can conform toan uneven surface on the adjacent heat source, effectively filling theslight voids resulting from the manufacturing process. Typical prior artpreformed films do not have good compressibility, which results in poorintimate contact between the surface of the heat sink and the heatgenerating source. Because of the printable nature of this material,variable screen or stencil sizes can be quickly made up when using thethermal interface material of the present invention. The film can beapplied during the heat sink manufacturing process and thus eliminatesthe need for an additional step during assembly. The interface padformed by the material of the present invention is substantiallynoncorrosive and it will not creep or bleed-out under applied pressure.

In addition, the present invention is greatly advantageous over theprior art because it is visible light curable. This feature is animprovement over the prior art interfaces that require the use ofultra-violet (UV) wavelength light for the curing step. For UV curing aspecial light source is required and extensive safety measures need tobe employed to protect the operator from exposure to dangerous levels ofUV radiation. Visible light curing allows the film to be applied andcured using conventional light sources and eliminates the hazard for theoperator.

It is therefore an object of the present invention to provide athermally conductive interface construction for a heat sink assemblythat is substantially compressible and can fill the voids andimperfections on the surfaces of the heat sink and object to be cooled.

It is also an object of the present invention to provide a thermallyconductive interface construction for a heat sink assembly that has ahigh thermal conductivity.

It is an object of the present invention to provide a thermallyconductive interface construction for a heat sink assembly that iseasily applied to a wide array of heat sink applications, thuseliminating an additional assembly step.

It is a further object of the present invention to provide a thermallyconductive interface construction for a heat sink assembly that solvesthe problems associated with the prior art.

Other objects, features and advantages of the invention shall becomeapparent as the description thereof proceeds when considered inconnection with the accompanying claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The thermally conductive material composition of the present inventioncomprises a visible light curable elastomeric material, a catalyst, athermally conductive filler and a hydrocarbon solvent. In the preferredembodiment the visible light curable composition comprises by weightfrom about 35% to about 75% of a visible light curable elastomericmaterial, from about 0.5% to about 15% catalyst, from about 10% to about30% hydrocarbon solvent and from about 20% to about 70% conductivefiller. The amount of hydrocarbon solvent added is controlled by therequired viscosity of the resultant material in order to facilitatescreen or stencil printing while maintaining sufficient viscosity tostay in place on the substrate at the desired layer thickness withoutslump or sag prior to the curing operation.

The present composition is formed by first providing an elastomeric basepolymer matrix. The elastomeric material selected has visible lightcurable properties. This feature of the present invention is importantin that it is an improvement over the prior art. The use of visiblelight for curing has several advantages. Use of visible light for curingeliminates required use of specialized, high powered lamps withemissions in the UV range. In addition, this eliminates the need forelaborate enclosures around the lights to shield the operator and otherpersonnel on the manufacturing floor from UV radiation emissions. Incontrast, the present invention allows the curing process to occur understandard visible light that will not harm the operating personnel in theevent that they are exposed. However it is possible that a formulationof the present invention may be cured using UV light curing equipment.This feature may be desirable if a user already has existing UV lightcuring equipment and will obviate the nedd to purchase additionalequipment.

The selection of an elastomeric base polymer for the present inventionis another important aspect of the present invention. Since the finishedproduct will have the characteristics found in elastomers, it will havemuch greater flexibility and compressibility as compared to the priorart thermal transfer interface products. In the heat transfer industrythe electronic devices being served continue to produce greater andgreater amounts of heat while the space allowed for cooling solutions iscontinually being reduced. As a result, smaller, thinner, highlyefficient and precise solutions must be developed. The prior art usedfor manufacturing such heat transfer and dissipation systems does notprovide the precision required to allow highly efficient heat transfer.The component parts used for heat transfer are either machined or diecast metals or injection molded, thermally conductive plastics. Thesurfaces of these materials generally have small imperfections such asmilling marks that result in small gaps between the mating surfaces.Since these gaps contain air the resultant thermal transfer between theheat generating device and the heat dissipating device is reduced. Theelastomeric material allows for a flexible and very compressibletransfer interface to be installed on the heat sink that conforms to thesurface imperfections eliminating the gaps and providing efficient heattransfer.

The elastomeric base material is loaded with a thermally conductivefiller material to facilitate thermal conductivity throughout thefinished composition. Thermally conductive fillers suitable for use inthe present invention include, for example, particles of boron nitride,the preferred filler in this invention, but also include aluminumnitride and alumina, as well as carbon materials such as carbon fiber.Mixtures of such materials may also be utilized. It will be appreciatedthat the particular conductive filler that is utilized is generally afunction of the particular application for the conductive compositionincluding for example, the amount of heat that must be transferred fromthe heat generating electronic device.

The present invention also employs a catalyst to allow the compositionto cure to a solid state after being applied to the desired surface. Apreferred catalyst is selected from the group consisting of1-hydroxycyclohexyl phenyl ketone,2-methyl-1-4-(methylthio)phenyl!-2-morpholinopropan-1-one, benzophenone,2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone,2,2-dimethoxy-2-phenyl acetophenone,bis(2,6-dimethoxybenzoyl-2,4-,4-trimethyl pentyl) phosphine oxide,2-hydroxy-2-methyl-1-phenyl-propan-1-one,2-hydroxy-2-methyl-1-phenyl-1-propane,2,4,6-trimethylbenzoyldiphenyl-phosphine oxide, 2-hydroxy2-methyl-1-phenyl-propan-1-one, mixed triaryl sulfoniumhexafluoroantimonate salts, mixed triaryl sulfonium hexafluorophosphatesalts, visible light photoinitiators. dl-camphorquinone, andcombinations thereof. The viscous mixture is applied to the desired heattransfer surface using either a screen printing or stencil printingprocess and is subsequently exposed to a source of visible light to curethe material. This leaves a compressible film on the heat transfersurface of the heat transfer device that has the desired thickness andflexibility while remaining in place throughout shipment and finalassembly of the device in a finished product.

Finally, the composition also contains a hydrocarbon solvent. Such ahydrocarbon solvent that can be used is an aromatic hydrocarbon solventsold under the trade designation Aromatic 100 by Union Carbide ofDanbury, Conn. Another suitable hydrocarbon solvent includes the ISOPARseries of solvents sold by Exxon Chemical of Houston, Tex. The amount ofsolvent employed in mixing the material is selected depending on theviscosity required for effective printing. The present invention isintended to be used in a screen or stencil printing operation, whereby asubstantial film thickness is developed on the surface of the partbefore the curing operation. The material composition must be thinenough to allow printing but thick enough to maintain a film thicknesswithout sagging or slumping prior to curing. In addition, the solventserves to facilitate the complete wetting of the conductive fillers. Thesolvent also allows high loading of the fillers in the materialcomposition. High loading of the fillers provides superior heat transferproperties as well as superior physical properties.

Reaction (crosslinking) of the elastomeric polymer materials isinitiated upon exposure to hig intensity visible light. Initiation ofthe free radical polymerization occurs upon absorption of light by thecatalysts or initiators. In addition, thermal catalysts or initiatorshelp to perpetuate the crosslinking reaction in shadow covered areas orareas blocked by the conductive filler. The curing parameter of thematerial composition depends on factors such as applied thickness,environmental conditions and energy levels of the light source. However,the material composition generally cures in a few seconds.

While there is shown and described herein certain specific structureembodying the invention, it will be manifest to those skilled in the artthat various modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the underlying inventiveconcept and that the same is not limited to the particular forms hereinshown and described except insofar as indicated by the scope of theappended claims.

What is claimed is:
 1. A method of forming a thermally conductive filmupon a substrate, said film having a thermal conductivity of at leastabout 1.5 W/m°K comprising the steps of: providing a visible lightcurable elastomeric base matrix and a catalyst material therein; mixinga thermally conductive filler material into said elastomeric base matrixand said catalyst to form a viscous mixture; thinning said viscousmixture using a hydrocarbon solvent; printing said thinned viscousmixture onto a heat dissipating substrate to form a film layer; andsubjecting said film layer to a source of visible light thereby curingsaid composition into a conductive film layer.
 2. The method as setforth in claim 1, wherein said step of printing said thinned viscousmixture onto a heat dissipating substrate is printing onto a heat sink.3. The method as set forth in claim 1, wherein said step of printingsaid thinned viscous mixture is screen printing.
 4. The method as setforth in claim 1, wherein said step of printing said thinned viscousmixture is stencil printing.
 5. The method as set forth in claim 1,wherein said step of mixing a thermally conductive filler material intosaid elastomeric base matrix further comprises mixing into said basematrix a thermally conductive filler material selected from the groupconsisting of boron nitride, aluminum nitride, alumna and mixturesthereof.